1. Field of the Invention
The present invention relates to an evaporation source for evaporating an organic electroluminescent layer. In particular, the present invention relates to the evaporation source preventing an aperture, through which a vaporized evaporation material is emitted, from being clogged by restricting heat transfer to outward.
2. Description of the Related Art
Thermal, physical vacuum evaporation is a technique for forming an organic electroluminescent layer on a substrate by emitting a vaporized evaporation material (organic material). In this evaporation process, an evaporation material retained in a vessel is heated to evaporation temperature, and after emitted from the vessel, the vaporized evaporation material is coated on the substrate. This process is carried out in a chamber whose pressure is maintained between 10−7 and 10−2 Torr, wherein the vessel retaining the evaporation material and the substrate is installed in the chamber.
Generally, the evaporation source, which is the vessel retaining the evaporation material, is made of an electrical resistance material, wherein the temperature of the electrical resistance material increases when the electric current flows through the walls of the evaporation source. When the electric current is applied to the evaporation source, the evaporation material retained therein is heated by radiation heat and conduction heat transferred from the walls of the evaporation source. An aperture for emitting the vaporized evaporation material to outward is formed on the upper surface of the evaporation source.
FIG. 1 is a sectional view showing the inner configuration of the vacuum evaporation apparatus equipped with a conventional evaporation source. The evaporation source 1 is installed in the chamber 3 of the evaporation apparatus, and the substrate 2 is placed above the evaporation source 1.
The substrate 2 on which the organic electroluminescent layer is evaporated is mounted on an upper plate 3-1 of the chamber 3, wherein the substrate 2 can be fixed or installed to move widthwise. A general configuration of the vacuum evaporation apparatus is to mount the substrate 2 on the upper plate 3-1 to move horizontally, and thus the explanation about this configuration is omitted.
The evaporation source 1 is installed on an insulated structure 4 fixed to a base 3-2 of the chamber 3, and connected to a cable for supplying electric power. The evaporation source 1 is capable of moving horizontally widthwise as well as being fixed to the insulated structure 4. Another general configuration of the vacuum evaporation apparatus is to install the evaporation source 1 on the insulated structure 4 to move horizontally, and thus the explanation about this configuration is also omitted.
The aperture 1A-1 formed on the upper surface of the evaporation source 1 is shown in FIG. 1, wherein the evaporation material vaporized in the evaporation source 1 is emitted through the aperture 1A-1 to outward in the direction of the substrate 2. Generally, the evaporation sources are classified into point evaporation source and linear evaporation source depending on the shape of evaporation source and aperture.
The entire shape of the point evaporation source is cylindrical, and the shape of its aperture is circular. The entire shape of the linear evaporation source is hexahedral, and the shape of its aperture is rectangular.
The selection of evaporation source is determined by considering the conditions of evaporation process and substrate, and the shape of evaporation layer to be formed. For convenience's sake, the point evaporation source will be explained below.
FIG. 2 is a sectional view showing a conventional point evaporation source. The point evaporation source 1 comprises a cell 1C, a base 1D and a cell cap 1A. The evaporation material, which is an organic material, is retained in the inner space formed by the cell 1C, the base 1D and the cell cap 1A.
A heating means 1B-1, for example, an electric resistance coil connected to electric power, is placed between the cell 1C and an external wall 1B to heat the evaporation material M retained in the inner space. The heating means 1B-1 is installed for the cell 1C of the entire height to heat the entire evaporation material M.
A cell cap aperture 1A-1 is formed in the center of the cell cap 1A, wherein the vaporized evaporation material M heated by the heating means 1B-1 is emitted through the cell cap aperture 1A-1 to outward, that is, to the direction of the substrate 2.
The temperature around the cell cap aperture 1A-1 is lower than the temperature of the inner space in which the vaporized evaporation material is generated because the cell cap 1A has no additional heating means installed thereon, and is exposed to the outside. Therefore, a part of the vaporized evaporation material emitted through the cell cap aperture 1A-1 is deposited around the cell cap aperture 1A-1 due to lower temperature thereabout.
As the evaporation process continues, the amount of the deposited evaporation material increases. Therefore, fluent emission of the vaporized evaporation material is not carried out, and in the end, the cell cap aperture 1A-1 is clogged by increase of the deposited evaporation material.
In order to prevent the vaporized evaporation material from being deposited around the cell cap aperture 1A-1, it is necessary that the temperature of the cell cap aperture 1A-1 or the cell cap 1A should keep above a predetermined temperature. Therefore, in order to do so, a cover 1E, made of metallic material, is mounted on the upper end of the external wall 1B as shown in FIG. 2, wherein the shape of the cover 1E is of a circular plate.
The cover 1E mounted on the upper end of the external wall 1B is placed on the cell cap 1A, and maintains a predetermined space from the cell cap 1A. A cover aperture 1E-1 for emitting the vaporized evaporation material is formed on the cover 1E to correspond to the cell cap aperture 1A-1. Therefore, the cell cap 1A may maintain a predetermined temperature because the cover 1E prevents the heat transferred from the cell cap 1A from being emitted to outward.
However, the heat transferred from the cell cap 1A is emitted to outward because the cover 1E is metallic. Therefore, the cell cap 1A cannot maintain a predetermined temperature, and so the deposition of the vaporized evaporation material around the cell cap aperture 1A-1 cannot be completely prevented.